Biosynthesis of alginate. 3. Tritium incorporation with polymannuronic acid 5-epimerase from Azotobacter vinelandii

Incubation of polymannuronic acid in tritiated water with the polymannuronic acid C-5-epimerase isolated from Azotobacter vinelandii led to incorporation of tritium into the glycuronan. Hydrolysis of the labelled glycuronan and separation of the uronic acids demonstrated that 92% of the activity was present in the guluronic acid produced by the epimerase. There was also a small, but significant, incorporation into the mannuronic acid. The results are discussed in relation to the present knowledge of the enzymic epimerisations, and it is suggested that the first step in the reaction is an abstraction of H-5.